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Laboratory Astrophysics: from Observations to Interpretation Proceedings IAU Symposium No. 350, 2019

F. Salama & H. Linnartz, eds. doi:10.1017/S1743921319006422

Gas-phase UV cross sections of radicals

A. N. Heays

1,2

, N. de Oliveira

3

, B. R. Lewis

4

, G. Stark

4

,

J. R. Lyons

2

, M. C. van Hemert

6

and E. F. van Dishoeck

7

1NASA Astrobiology Institute

2ASU School of Earth and Space Exploration, USA 3Synchrotron SOLEIL, France

4Australian National University 5Wellesley College MA, USA

6Leiden Institute of Chemistry, Leiden University, the Netherlands 7Leiden Observatory, Leiden University, the Netherlands

Abstract. We present some new studies of the absolute photodissociation cross sections of astrophysically-relevant diatomic radicals.

Keywords. Astrochemistry, diatomic molecules, photodissociation, Methods: laboratory, theory

1. Introduction

A century of classical electronic spectroscopy has provided quantitative photoprocess cross sections spanning an enormous number of species and wavelength ranges. But chal-lenges remain in the far-, vacuum-, and extreme-ultraviolet wavelengths where radical species are concerned. This is due to the difficulty of obtaining measurable column den-sities and the complexity of predicting overlapping high-energy states. Another difficulty with studying open-shell molecules is that fine structure of both ground and excited states leads to a complicated overlap of rotational lines. This problem is worsened where the excited state is heavily predissociated and lifetime broadening leads to further line confusion.

Nonetheless, the quantitative interpretation of ongoing remote and in-place obser-vations of ultraviolet-irradiated planetary atmospheres and interstellar gas relies on accurate photoabsorption and photodissociation cross sections, not all of which are known. One case where fine isotope-dependent details count is in geochemical or remote observations of isotope fractionation where this can be traced to a photolytic origin.

Below is a summary of four studies we have made of atmospherically- or astrophysically-relevant radical cross sections to address data needs in the astrochemical and photochemical communities.

2. Results

2.1. OHD ← X

This molecule is widely observed in the interstellar medium and cometary comae where it is itself a photolysis product of water. Its photodissociation cross sections is well-studied theoretically (van Dishoeck & Dalgarno(1984)) but not measured experimentally. Using the far-ultraviolet-capable Fourier-transform spectrometer and bright synchrotron

c

 International Astronomical Union 2020

available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921319006422

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438 A. N. Heays et al.

Figure 1. The new photodissociation cross sections. OH : Effective Hamiltonian model of the

D(0) ← X(0) band. SO: Global model of interacting B(vB) ← X(0) and C(vC) ← X(0) bands.

S2: Experimental cross section at 370 K. CN : Ab initio electronic-vibrational spectrum.

radiation source at the DESIRS beamline of the SOLEIL synchrotron (de Oliveira et al.

(2016)) we recorded two photoabsorption bands of the D2Σ+← X2Π system (Heays et al.(2018)).

2.2. CN highly-excited states

CN is another astrophysically prevalent molecule in irradiated environments where its lifetime is limited by photodissociation of its highly-excited unbound states. We performed a new quantum-chemical calculation of these states and their absorption properties employing a CASSCF calculation with as large an active-space as feasible followed by multi-reference configuration interaction computation. The challenge of this calculation arises due to the high density of vacuum-ultraviolet accessible doublet-states approaching the 91.2 nm Lyman-limit, due to the relatively low ionisation threshold of CN. A further computation found spin-orbit predissociation of low-lying bound excited states through a nonradiative intersystem transition into unbound quartet states. This leads to a weak photodissociation cross section in the far-ultraviolet which is nonetheless significant in long-wavelength dominated radiation fields.

2.3. S2 B ← X

Photoabsorption by theB(v) ← X(0) bands dominates dissociation of S2. This radical was generated as sulphur vapour in an oven at around 800 K and in a microwave discharge nearer room temperature (Stark et al.(2018)). Fourier-transform absorption spectroscopy was use to measure cross sections between 240 and 285 nm. This data was used to con-strain a coupled-channels model of theBu state and its predissociation (Lewis et al.

(2018)). This predissociation is evident in the experimental cross section shown in Fig.1

where only the longest-wavelength electronic-vibrational band shows clear rotational structure.

available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921319006422

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Gas-phase UV cross sections of radicals 439 2.4. SO B ← X and C ← X

Fourier-transform spectra of SO generated in a microwave discharge through SO2 was recorded for four isotopologues. An experimental difficulty in this case arose from heavy contamination by parent SO2. Isotope-independent potential-energy curves and a B3Σ∼ C3Π spin-orbit interaction was deduced from this experimental data. A 380 K

photodissociation cross section for32S16O computed by this model is shown in Fig.1. References

de Oliveira, N., Joyeux, D., Roudjane, M., et al. 2016, J. Synchrotron Radiat., 23, 887 Heays, A. N., de Oliveira, N., Gans, B., et al. 2018, JQSRT, 204, 12

Lewis, B. R., Gibson, S. T., Stark, G., & Heays, A. N. 2018, J. Chem. Phys., 148, 244303 Stark, G., Herde, H., Lyons, J. R., et al. 2018, J. Chem. Phys., 148, 244302

van Dishoeck, E. F. & Dalgarno, A. 1984, J. Chem. Phys., 277, 576

available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921319006422

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